ROME — In a combative letter, a highly placed cardinal on Sunday mounted the Vatican’s first direct response to accusations that Pope Francis knew about and covered up the alleged sexual misconduct of a U.S. prelate, describing those claims as a “political plot that lacks any real basis.”

The letter, written by Canadian Cardinal Marc Ouellet, came six weeks after a former Vatican ambassador to the United States wrote a bombshell letter of his own, charging that much of the Vatican hierarchy, including Francis, had for years protected recently resigned cardinal Theodore McCarrick.

Ouellet’s letter is significant because it ends a period of overwhelming silence among the key Vatican officials with the standing to rebut or back up the claims of that former ambassador, Carlo Maria Viganò. That silence has tested the patience of many Catholics, who remain divided over Viganò’s credibility but say his claims have further wounded a church that is contending with multiple abuse-related crises.

Ouellet, the head of the Vatican’s powerful bishops office, said it was “unbelievable and without any foundation” to accuse Francis of “having covered-up knowingly the case of an alleged sexual predator.” Ouellet, who portrayed Viganò as bitter and disillusioned with his career within the Holy See, said he was in “open and scandalous rebellion.” Ouellet accused him of exploiting the broader clergy sexual abuse scandal in the United States as a way to land “an unmerited and unheard of blow” on the pope.

But in pushing back against Viganò, Ouellet said the Vatican had attempted years ago to place some restrictions on McCarrick — an acknowledgment that matches somewhat, but not completely, with Viganò’s version of events.

As Viganò describes it, the Vatican had ignored reports about McCarrick for years — until “2009 or 2010,” when Pope Benedict XVI placed sanctions on the cardinal, forbidding him to travel, appear publicly or hold Mass. Viganò says he then told Francis in 2013, not long after he became pope, that McCarrick was a “serial predator.” Francis nonetheless reportedly ignored those sanctions and made McCarrick a trusted adviser.

Viganò wrote that he learned of Benedict’s sanctions against McCarrick in part from Ouellet.

But as Ouellet describes it in his letter, which was addressed to Viganò, the Vatican’s measures against McCarrick did not reach the papal level. At an unspecified time, Ouellet writes, McCarrick was “requested not to travel or to make public appearances, in order to avoid new rumors about him.” The measures could have been stronger, Ouellet said, had the Vatican been supplied with clear evidence of McCarrick’s misconduct by its representatives — including Viganò — working in the United States.

“It is false, therefore, to present those measures as ‘sanctions’ formally imposed by Pope Benedict XVI and then invalidated by Pope Francis,” Ouellet wrote. “After a review of the archives, I find that there are no documents signed by either pope in this regard.”

Though the Roman Catholic Church is dealing with abuse scandals in multiple countries, McCarrick’s case has become among the most damaging because it raises questions about who within the Vatican’s hierarchy knew of the prelate’s behavior as he rose to become one of the most prominent figures within the global church. McCarrick is accused of sexual abuse of adults and minors, and in July, he became the first cardinal in nearly a century to fully resign his position.

He is 88, and in 2006, when McCarrick reached the customary retirement age of 75, Benedict accepted his retirement as the archbishop of Washington.

McCarrick’s case has battered Francis’s reputation, but some Vatican watchers say the Holy See’s promised “study” into McCarrick’s case, which it announced Saturday, could shine a spotlight on the actions of previous popes, including Benedict and John Paul II — who promoted McCarrick through the ranks and elevated him to cardinal in 2001.

Ouellet, who became head of the bishops office in 2010, said he was “very surprised” by McCar­rick’s rise and wrote that he recognized the “failures in the selection procedures implemented in his case.” Ouellet specifically defended Francis and said — ticking off the stops in McCarrick’s career — that the current pope “had nothing to do with McCar­rick’s promotions to New York, Metuchen, Newark and Washington.”

“It is clear this [McCarrick case] had its origins in 1980s, 1990s, early 2000s,” said Massimo Faggioli, a professor of theology and religious studies at Villanova University. “The problem is that this could be embarrassing for a pope who is dead and a saint, and even more embarrassing for Pope Benedict, because he’s still alive.”

Viganò says he told Francis about McCarrick’s behavior with young men on June 23, 2013. Ouellet does not directly reject this claim but notes that the pontiff that day met with a long line of Vatican representatives. “I strongly doubt that the Pope had such interest in McCarrick, as you would like us to believe,” Ouellet wrote, saying that McCarrick was 82 at the time and had stepped down from his duties seven years earlier.

After Viganò’s letter was released by several conservative Catholic outlets in late August, Francis vowed not to “say a single word on this.” But he made several opaque remarks that some interpreted as being aimed at Viganò, at one point saying during a homily that a “Great Accuser,” Satan, is “among us.”

Other cardinals mentioned in Viganò’s letter refused to comment, and one, Cardinal Giovanni Battista Re, Ouellet’s predecessor as head of the bishops office, attributed his silence to Francis.

“The pope has chosen not to talk, so it’s not as if I can,” Re said. “So, I’m sorry.”

Stefano Pitrelli contributed to this report.